Understanding the Oxygen Activities in High Voltage Cathode Materials in Lithium and Sodium Ion Battieres

Among all candidates of positive electrode materials for lithium ion batteries, the lithium-excess layered oxides have gained growing research interest in recent years. They can reach more than 290mAh/g capacity with an average voltage of 4 V, making them the positive electrode material with highest energy density among all known intercalation compounds for cathode materials. Being the source of major capacity improvement, the extra lithium ions, on the other hand, also cause unexplained phenomena in lithium-excess layered oxides. The rate performances of these materials are usually poor, and electron sources for the extra capacities have been under intense debate for years. In classical layered oxides, the transition metal (TM) oxide host TMO₂ should retain its host structure during initial discharging–charging, while lithium ions are intercalated–de-intercalated from the host. In contrast, recent studies suggested that in lithium-excess manganese rich layered oxides, certain amounts of transition metal ions migrate from the transition metal layers to lithium layers during the very first charging–discharging cycle. Our previous work has suggested that the migration may be assisted by oxygen vacancies generated in the late charging states of the first cycle, but three critical questions remain unanswered: (1) how can we visualize and quantify the change in structure and chemical content at the atomic level (2) what is the energy required for oxygen vacancies formation upon cycling and how do they assist transition metal migration and (3) are the oxygen loss reversible or partially reversible. A similar phenomena could be observed in sodium intercalation compounds. The oxygen activiites in the intercalation compounds at high voltage will be discuced in details.